During the operation of internal combustion engines which, for example, can be arranged in motor vehicles, so-called blow-by gas gets through a leakage between pistons and cylinders into a crankcase of the respective internal combustion engine. In order to avoid an unacceptable overpressure in the crankcase or an undesirable emission of blow-by gases into the environment, so-called crankcase ventilation devices are used. Usually, such a ventilation device connects, by means of a vent line, the crankcase to an intake passage of the internal combustion engine in which, in particular downstream of a throttle valve, a relative negative pressure prevails which enables the blow-by gas accumulating in the crankcase to be sucked out of said crankcase. Furthermore, during the operation of the internal combustion engine, an oil mist is generated which is aerosolically dissolved and mixes with the blow-by gas. In order to reduce the oil consumption of the internal combustion engine and also the pollutant emission, usually an oil mist separator is used which separates the oil particles aerosolically dissolved in the blow-by gas and feeds said particles to an adequate oil reservoir. An increasing tightening of pollutant emission regulations requires a steady improvement of the cleaning effect of such oil mist separators.
From EP 0 860 589 B1, for example, an oil mist separator for a crankcase ventilation device is known which comprises a first and a second separator unit and a bypass valve.